Cholesterol is found in nearly all plant and animal cells, either in its free form or in an ester form. Free cholesterol refers to cholesterol in its unreacted state (.DELTA..sup.5 -cholesterol, or cholest-5-en-3.beta.-ol). Total cholesterol refers to the sum of free cholesterol and its ester derivatives such as the linoleate, oleate, palmitate, arachidonate, palmitoleate, linolenate, stearate, and myristate esters. Cholesterol is found in constant amounts in serum under normal conditions. In general, 25% of the total cholesterol level in serum is free cholesterol while the remaining 75% is in the form of ester derivatives.
It is fairly well established that the total cholesterol content of whole blood is directly related to certain maladies in man and animals. Among the many maladies which have been found to be related to total cholesterol levels in blood are hepatocellular diseases, thyroid metabolism disorders, biliary obstruction and perhaps most importantly atherosclerosis and other vascular difficulties. Until the last decade it was customary to determine the total cholesterol content of whole blood, but it is now known that the serum level is altered by factors which do not affect the red cell level. As a result, most clinical determinations of both free and total cholesterol are performed on serum rather than on whole blood.